For safety and environmental protection reasons, motor-vehicle manufacturers require fuel tubes to have good mechanical properties, such as burst strength and flexibility, good cold (−40° C.) impact strength and high-temperature strength, a very low permeability to hydrocarbons and to their additives, particularly alcohols such as methanol and ethanol. These tubes must also have good resistance to the fuels and lubrication oils for engines. These tubes are manufactured by co-extruding the various layers using standard techniques for thermoplastics.
One type of tube used in these applications is based on a polyamide/fluoropolymer structure, such as in European patent number EP 558 373. The petrol permeability is excellent but the shock resistance is insufficient.
Multi-layer tubes, such as described in EP 696 301, EP 740 754 and EP 726 926 disclose tubes for transporting fuel, which comprise respectively a polyamide outer layer, a tie layer, a PVDF (polyvinylidene fluoride) layer, a tie layer and a polyamide inner layer in contact with the petrol. Adhesion between PVDF and polyamides is difficult due to the inertness of the PVDF.
Ethyl vinyl alcohol (EVOH) and polyvinyl alcohol (PVOH) can be used as a less expensive barrier layer. It also has a density differential advantage, and can provide a better permeation barrier for many hydrocarbon components.
One difficulty with multi-layer tubing is that the thickness of the layers and the unsymmetrical design can produce undesired stress during manufacture and use. These stresses can lead to splitting, resulting in serious safety concerns for low permeable multi-layer fuel tubing.
Surprisingly, it has now been found that by combining more flexible polyamide inner and outer layers with an EVOH or PVOH layer, with a symmetrical design, the safety issues caused by splitting of a multi-layer tubing can be resolved.